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US4639304A - Apparatus for determination of aluminum oxide content of the cryolite melt in aluminum electrolysis cells - Google Patents

Apparatus for determination of aluminum oxide content of the cryolite melt in aluminum electrolysis cells Download PDF

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Publication number
US4639304A
US4639304A US06/794,103 US79410385A US4639304A US 4639304 A US4639304 A US 4639304A US 79410385 A US79410385 A US 79410385A US 4639304 A US4639304 A US 4639304A
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Prior art keywords
aluminium
cryolite
electrode
melt
alumina
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Expired - Fee Related
Application number
US06/794,103
Inventor
Imre Bader
Endre Berecz
Gabor Szina
Janos Horvath
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Nehezipari Muszaki Egyetem
Magyar Aluminiumipari Troeszt
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Nehezipari Muszaki Egyetem
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Priority to HU831702A priority Critical patent/HU191839B/en
Priority to DE19853535754 priority patent/DE3535754A1/en
Priority to FR858516134A priority patent/FR2589579B1/en
Application filed by Nehezipari Muszaki Egyetem filed Critical Nehezipari Muszaki Egyetem
Priority to US06/794,103 priority patent/US4639304A/en
Assigned to NEHEZIPARI MUSZAKI EGYETEM, MAGYAR ALUMINIUMIPARI TROSZT reassignment NEHEZIPARI MUSZAKI EGYETEM ASSIGNMENT OF ASSIGNORS INTEREST. Assignors: BADER, IMRE, BERECZ, ENDRE, HORVATH, JANOS, SZINA, GABOR
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    • G—PHYSICS
    • G01—MEASURING; TESTING
    • G01N—INVESTIGATING OR ANALYSING MATERIALS BY DETERMINING THEIR CHEMICAL OR PHYSICAL PROPERTIES
    • G01N27/00—Investigating or analysing materials by the use of electric, electrochemical, or magnetic means
    • G01N27/26—Investigating or analysing materials by the use of electric, electrochemical, or magnetic means by investigating electrochemical variables; by using electrolysis or electrophoresis
    • G01N27/403—Cells and electrode assemblies
    • G01N27/406—Cells and probes with solid electrolytes
    • G01N27/411—Cells and probes with solid electrolytes for investigating or analysing of liquid metals
    • G01N27/4112—Composition or fabrication of the solid electrolyte
    • G01N27/4114—Composition or fabrication of the solid electrolyte for detection of gases other than oxygen

Definitions

  • the invention relates to an apparatus for determination of aluminium oxide content of the cryolite melt in aluminium electrolysis cells comprising an oxygen ion conducting solid electrolyte containing oxygen galvanic cell equipped with a reference electrode of given oxygen potential arranged in a zirconium oxide tube closed on one end and with a measuring electrode covered with aluminium and being in connection with the cryolite melt.
  • Measuring of the alumina in an aluminium electrolysis cell is done almost exclusively by classical analytical methods after sampling. This is very slow and cumbersome and therefore there is no chance to intervene quickly in the technology.
  • the aluminium oxide content of cryolite melts has been evaluated by electrochemical methods. Such a procedure is described in the Hungarian Pat. No. 175438. In this case the electrolytic potential is increased on a graphite electrode of known surface and this results in the increase of the current density.
  • the Al 2 O 3 concentration of the cryolite melt can be calculated from the actual anodic marginal current. This is an intermittent method and the data obtained are characteristic only in a short time interval.
  • the apparatus described in the German patent applications No. 1 798 248, No. 1 798 307 and No. 23 50 485 measure the oxygen content of the melts by means of oxygen ion conducting solid electrolyte containing oxygen galvanic cells.
  • the apparatus contain zirconium oxide tubes with reference electrodes therein.
  • the electrodes are materials of different oxygen potential, for example, a mixture of metal and metal oxide; gases, for example air, etc.
  • the measuring electrode is a conductor usually fitted onto the outer surface of the zirconium oxide tube or placed beside it. The conductor has a covering which makes possible the transmission of electrons.
  • the object of the present invention is to provide an apparatus which measures simply and continously the aluminium oxide content of the cryolite melt in the electrolysis cell, the preparation of which is simple and contains reliable electrodes.
  • the apparatus according to the invention is an oxygen ion conducting solid electrolyte containing oxygen galvanic cell equipped with a reference electrode of given oxygen potential arranged in a zirconium oxide tube closed on one end and with a measuring electrode covered with aluminium and being in connection with the cryolite melt.
  • the reference electrode according to the invention is cryolite melt supersaturated with Al 2 O 3 while the aluminium coating of the measuring electrode is the molten aluminium of the furnace into which the electrode is immersed.
  • the apparatus can be used under technological conditions as a probe which provides high accuracy and comfort even at high temperature and in a contaminated environment.
  • FIG. 1 shows the schematic illustration of the apparatus under technological conditions.
  • FIG. 2 shows the diagram obtained by means of the apparatus.
  • the measuring electrode /9/ can be seen in the form of a conductor /11/ covered with a protective coating /10/.
  • thermocouple /12/ measuring the temperature is located.
  • thermocouple /12/ The elctrodes /7 and 9/ and the thermocouple /12/ are connected to the same instrument /13/.
  • the solid electrolyte /6/ which contains the reference electrode /7/ is in zirconium oxide tube closed at one end forming this way an oxygen ion permissive container.
  • the cryolite and the appropriate amount of alumina is poured into this container.
  • the appropriate amount means that the cryolite melt should be supersaturated with Al 2 O 3 . It can be realized very simply with the overcharge of the alumina. This way adjustment and control are not necessary.
  • the current supplying wire /8/ protrudes into the so obtained reference electrode /7/ and, at the other end it is connected to the instrument /13/.
  • the solid electrolyte /6/ is fitted by its upper part to a boron nitride tube which is cryolite-resistant.
  • the so obtained electrode can work for more hours even in a very corrosive melt which is characteristic for the alumina electrolysis. Therefore it is suitable for continous measuring.
  • the lead-in wire /11/ covered with the protective coating /10/ forms the measuring electrode /9/, where the lead-in wire /11/ is made of molybdenum.
  • the protective coating /10/ is made of boron nitride, thus also the measuring electrode /9/ is resistant to corrosion.
  • the present invention provides an apparatus being a galvanic cell, where with alumina saturated cryolite melt and the aluminium forms the reference electrode and the aluminium melt itself forms the measuring electrode as the measuring electrode /9/ protrudes into the molten aluminium /2/.
  • the reference electrode /7/ and the thermocouple /12/ are placed in the cryolite-alumina bath /3/ layer.
  • E MF the measured electromotive force /V/
  • E° the electromotive force /V/ in the 1 m % cryolite melt at T temperature
  • FIG. 2 shows the diagram obtained under technological conditions. This Figure shows how E MF changes versus time when under intensive stirring we increase the aluminium oxide content of the 1 m.% aluminium oxide containing cryolite melt by adding additionally aluminium oxide. Because of the sensitive probe we observe a sudden change of electromotive force after adding 1% Al 2 O 3 to the cryolite melt. After dissolution of alumina a steady state develops, the level of which is higher than before the peak. Adding 2% aluminium oxide the reaction is analogous but the difference between the level of the steady state e.m.f /because of the less concentration difference between the parts divided by the ZrO 2 probe/ will be less.
  • the measuring of this difference is sufficient, because it makes it unambigously possible to determine whether the alumina concentration in the electrolysis cell is low /2-3%/ or high /5-6%/.
  • the alumina concentration in the electrolysis cell quantitatively. Knowing the calibration curve relating to the e.m.f and the concentration of aluminium oxide the unknown concentration can be measured.
  • the above example shows that the apparatus according to the invention can be used simply, safely, relatively long lasting and continously for the determination of the aluminium oxide content of the cryolite melt.
  • the main advantage of the invention is the simple construction of electrodes.
  • the so obtained electrode can be prepared on the spot safely, very quickly without any measuring.
  • the preparation of the measuring electrode is less difficult, it is enough to immerse the current supplying wire covered with protective coating into the aluminium melt and the electrode works.
  • the measuring of the spatial inhomogenities of the aluminium oxide at different points in the electrolyte helps to develop the optimum construction of the electrolyzer cell and to determine the optimum place of input of aluminium oxide.
  • the speed of measuring enables to observe the dissolution of aluminium oxide in the cryolite after the crust-breaking and alumina charging. It is possible this way to find the technologically most appropriate quality of alumina

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Abstract

An analytical apparatus for determining the cryolite concentration in a cryolite-alumina electrolysis cell. The apparatus uses a reference electrode comprising a solid oxygen electrolyte tube containing a reference electrode of a cryolite melt supersaturated with alumina. A measuring electrode is combined with such a reference electrode to provide a potential representative of alumina concentration in the alumina electrolysis cell.

Description

BACKROUND OF THE INVENTION
The invention relates to an apparatus for determination of aluminium oxide content of the cryolite melt in aluminium electrolysis cells comprising an oxygen ion conducting solid electrolyte containing oxygen galvanic cell equipped with a reference electrode of given oxygen potential arranged in a zirconium oxide tube closed on one end and with a measuring electrode covered with aluminium and being in connection with the cryolite melt.
It is known that the current efficiency can be inreased by keeping the appropriate alumina concentration in the course of aluminium electrolysis and, thus, it is an important economical aspect.
Measuring of the alumina in an aluminium electrolysis cell is done almost exclusively by classical analytical methods after sampling. This is very slow and cumbersome and therefore there is no chance to intervene quickly in the technology. The instrumental analytical methods--x-ray diffraction and microscope--works only on the basis of a previous sampling.
The aluminium oxide content of cryolite melts has been evaluated by electrochemical methods. Such a procedure is described in the Hungarian Pat. No. 175438. In this case the electrolytic potential is increased on a graphite electrode of known surface and this results in the increase of the current density. The Al2 O3 concentration of the cryolite melt can be calculated from the actual anodic marginal current. This is an intermittent method and the data obtained are characteristic only in a short time interval.
Further disadvantage of this method is that the use of a polarizing unit is necessary for the operation and polarization programs should be started and run in each test.
The apparatus described in the German patent applications No. 1 798 248, No. 1 798 307 and No. 23 50 485 measure the oxygen content of the melts by means of oxygen ion conducting solid electrolyte containing oxygen galvanic cells.
These apparatus contain zirconium oxide tubes with reference electrodes therein. The electrodes are materials of different oxygen potential, for example, a mixture of metal and metal oxide; gases, for example air, etc. The measuring electrode is a conductor usually fitted onto the outer surface of the zirconium oxide tube or placed beside it. The conductor has a covering which makes possible the transmission of electrons.
In spite of its simplicity, this apparatus is suitable for continous measuring. Its disadvantage is, however, that it is difficult to keep the advantageous oxygen potential of the reference electrode. In case of gases the constant oxygen potential while in case of other reference materials the choice of material producing appropriate results and the assembly of the electrode are the problems.
There are difficulties also with the measuring electrode and with its covering especially in case of melts at high temperature. In case of cryolite melts the conductor has to be covered with aluminium, but at high temperature it gets damaged very quickly and forms more interfaces which cause inaccuracy in the measuring.
SUMMARY OF THE INVENTION
Therefore the object of the present invention is to provide an apparatus which measures simply and continously the aluminium oxide content of the cryolite melt in the electrolysis cell, the preparation of which is simple and contains reliable electrodes.
The apparatus according to the invention is an oxygen ion conducting solid electrolyte containing oxygen galvanic cell equipped with a reference electrode of given oxygen potential arranged in a zirconium oxide tube closed on one end and with a measuring electrode covered with aluminium and being in connection with the cryolite melt. The reference electrode according to the invention is cryolite melt supersaturated with Al2 O3 while the aluminium coating of the measuring electrode is the molten aluminium of the furnace into which the electrode is immersed.
Practically the conductor of the measuring electrode is covered with boron nitride protective coating.
The apparatus can be used under technological conditions as a probe which provides high accuracy and comfort even at high temperature and in a contaminated environment.
There are no special requirements to meet with the probe in order to provide accuracy, that means there is need to define exactly the surface of the measuring electrode, because the aluminium melt itself forms the surface and choosing the potential of this melt the average concentration of the aluminium oxide can be measured well.
BRIEF DESCRIPTION OF THE DRAWINGS
FIG. 1 shows the schematic illustration of the apparatus under technological conditions.
FIG. 2 shows the diagram obtained by means of the apparatus.
DESCRIPTION OF THE PREFERRED EMBODIMENTS
On the FIG. 1, on the bottom of an electrolysis cell /1/ there is the aluminium melt /2/, over it the cryolite-alumina melt (3) is placed while the slag /4/ is on the top. An anode /5/ protrudes into the electrolysis cell /1/. The apparatus according to the invention is shown under technological conditions. From the left a reference electrode /7/ and the current supplying wire /8/ can be seen, both are placed in a solid electrolyte tube /6/.
In the middle the measuring electrode /9/ can be seen in the form of a conductor /11/ covered with a protective coating /10/.
From the right a thermocouple /12/ measuring the temperature is located.
The elctrodes /7 and 9/ and the thermocouple /12/ are connected to the same instrument /13/.
The solid electrolyte /6/ which contains the reference electrode /7/ is in zirconium oxide tube closed at one end forming this way an oxygen ion permissive container. The cryolite and the appropriate amount of alumina is poured into this container. The appropriate amount means that the cryolite melt should be supersaturated with Al2 O3. It can be realized very simply with the overcharge of the alumina. This way adjustment and control are not necessary. The current supplying wire /8/ protrudes into the so obtained reference electrode /7/ and, at the other end it is connected to the instrument /13/. The solid electrolyte /6/ is fitted by its upper part to a boron nitride tube which is cryolite-resistant.
The so obtained electrode can work for more hours even in a very corrosive melt which is characteristic for the alumina electrolysis. Therefore it is suitable for continous measuring.
The lead-in wire /11/ covered with the protective coating /10/ forms the measuring electrode /9/, where the lead-in wire /11/ is made of molybdenum. The protective coating /10/ is made of boron nitride, thus also the measuring electrode /9/ is resistant to corrosion.
To sum up, the present invention provides an apparatus being a galvanic cell, where with alumina saturated cryolite melt and the aluminium forms the reference electrode and the aluminium melt itself forms the measuring electrode as the measuring electrode /9/ protrudes into the molten aluminium /2/. The reference electrode /7/ and the thermocouple /12/ are placed in the cryolite-alumina bath /3/ layer.
In the course of measuring it is possible to register the change of electromotive force and the Al3 O3 content is calculated from it with following formula:
E.sub.MF =E°-(RT)/(GF)1n/%Al.sub.2 O.sub.3 /
where
EMF =the measured electromotive force /V/
E°=the electromotive force /V/ in the 1 m % cryolite melt at T temperature
R=the molar gas constant /8,31433 J mol-1 K-1 /
F=Faraday constant /96487 C mol-1 /
FIG. 2 shows the diagram obtained under technological conditions. This Figure shows how EMF changes versus time when under intensive stirring we increase the aluminium oxide content of the 1 m.% aluminium oxide containing cryolite melt by adding additionally aluminium oxide. Because of the sensitive probe we observe a sudden change of electromotive force after adding 1% Al2 O3 to the cryolite melt. After dissolution of alumina a steady state develops, the level of which is higher than before the peak. Adding 2% aluminium oxide the reaction is analogous but the difference between the level of the steady state e.m.f /because of the less concentration difference between the parts divided by the ZrO2 probe/ will be less.
It can be seen that the change of the e.m.f. is minimal at 5% and 6%.
For the industrial point of view the measuring of this difference is sufficient, because it makes it unambigously possible to determine whether the alumina concentration in the electrolysis cell is low /2-3%/ or high /5-6%/.
Generally it is sufficient to know the alumina concentration in the electrolysis cell quantitatively. Knowing the calibration curve relating to the e.m.f and the concentration of aluminium oxide the unknown concentration can be measured.
The above example shows that the apparatus according to the invention can be used simply, safely, relatively long lasting and continously for the determination of the aluminium oxide content of the cryolite melt.
The main advantage of the invention is the simple construction of electrodes. The reference electrode--as it has been mentioned before--can be filled with the cryolite melt from the electrolysis cell and adding of alumina is necessary. The so obtained electrode can be prepared on the spot safely, very quickly without any measuring.
The preparation of the measuring electrode is less difficult, it is enough to immerse the current supplying wire covered with protective coating into the aluminium melt and the electrode works.
With this method the measuring of the spatial inhomogenities of the aluminium oxide at different points in the electrolyte helps to develop the optimum construction of the electrolyzer cell and to determine the optimum place of input of aluminium oxide.
The speed of measuring enables to observe the dissolution of aluminium oxide in the cryolite after the crust-breaking and alumina charging. It is possible this way to find the technologically most appropriate quality of alumina
While several embodiments of the invention have been shown and described, it will be apparent to those skilled in the art that modifications may be made therein without departing from the scope of the invention.

Claims (2)

What we claim is:
1. An apparatus for determination of aluminium oxide content of a cryolite melt in aluminium electrolysis cells comprising an oxygen ion conducting solid electrolyte containing oxygen galvanic cell equipped with a reference electrode of given oxygen potential arranged in a zirconium oxide tube closed on one end and with a measuring electrode covered with aluminium, said measuring electrode comprising a conductor, said reference electrode being a cryolite melt supersaturated with Al2 O3 and the aluminium coating of the measuring electrode being the aluminium melt in the cell into which the conductor of the electrode protrudes.
2. The apparatus as claimed in claim 1 wherein at least a portion of the the conductor of the measuring electrode has a boron nitride protective coating.
US06/794,103 1983-05-16 1985-11-01 Apparatus for determination of aluminum oxide content of the cryolite melt in aluminum electrolysis cells Expired - Fee Related US4639304A (en)

Priority Applications (4)

Application Number Priority Date Filing Date Title
HU831702A HU191839B (en) 1983-05-16 1983-05-16 Method and device for measuring continuously the solute alumina content of cryolite melts with alumina content during operation
DE19853535754 DE3535754A1 (en) 1983-05-16 1985-10-07 DEVICE FOR MEASURING THE ALUMINUM OXIDE CONTENT OF THE CRYOLITE MELT IN ALUMINUM ELECTROLYSIS OVENS
FR858516134A FR2589579B1 (en) 1983-05-16 1985-10-30 DEVICE FOR MEASURING THE ALUMINUM OXIDE CONTENT OF A CRYOLITE BATH FOUND IN AN ALUMINUM ELECTROLYSIS OVEN
US06/794,103 US4639304A (en) 1983-05-16 1985-11-01 Apparatus for determination of aluminum oxide content of the cryolite melt in aluminum electrolysis cells

Applications Claiming Priority (2)

Application Number Priority Date Filing Date Title
HU831702A HU191839B (en) 1983-05-16 1983-05-16 Method and device for measuring continuously the solute alumina content of cryolite melts with alumina content during operation
US06/794,103 US4639304A (en) 1983-05-16 1985-11-01 Apparatus for determination of aluminum oxide content of the cryolite melt in aluminum electrolysis cells

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HU (1) HU191839B (en)

Cited By (9)

* Cited by examiner, † Cited by third party
Publication number Priority date Publication date Assignee Title
US4921584A (en) * 1987-11-03 1990-05-01 Battelle Memorial Institute Anode film formation and control
EP0285578A3 (en) * 1987-03-27 1990-05-16 Centro Sviluppo Materiali S.P.A. Improvement of electrochemical devices for measuring the silicon content of hot metal
US4935107A (en) * 1986-10-17 1990-06-19 Aluminium Pechiney Process for electrochemical measurement of the concentration of oxide ions in a bath based on molten halides
US5294313A (en) * 1992-08-25 1994-03-15 The United States Of America As Represented By The United States Department Of Energy Sensors for monitoring waste glass quality and method of using the same
US5342489A (en) * 1989-10-17 1994-08-30 Yamari Electronite Kabushikigaisha Method of measuring oxygen activities in slag
US5989408A (en) * 1996-12-18 1999-11-23 Heraeus Electro-Nite International N.V. Process for measuring an electrochemical activity
US6010611A (en) * 1995-09-01 2000-01-04 Auckland Uniservices Limited Measurement of alumina in reduction pots
WO2000054040A1 (en) * 1999-03-05 2000-09-14 Heraeus Electro-Nite International N.V. Immersion sensor, set-up of measuring instruments and measuring method for monitoring aluminium electrolysis cell
US20170145574A1 (en) * 2014-06-19 2017-05-25 United Company RUSAL Engineering and Technology LLC Method for controlling an alumina feed to electrolytic cells for producing aluminum

Families Citing this family (1)

* Cited by examiner, † Cited by third party
Publication number Priority date Publication date Assignee Title
CN111272602A (en) * 2020-02-16 2020-06-12 南通市产品质量监督检验所 Method for measuring Al content of zinc-aluminum coating

Citations (1)

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US3471390A (en) * 1965-03-24 1969-10-07 Reynolds Metals Co Alumina concentration meter

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US3758397A (en) * 1970-07-15 1973-09-11 Steel Corp Apparatus for oxygen determination
US3668099A (en) * 1971-02-16 1972-06-06 United States Steel Corp Apparatus for measuring oxygen content of a fluid
DE2350485C2 (en) * 1973-10-08 1984-08-02 Interatom Internationale Atomreaktorbau Gmbh, 5060 Bergisch Gladbach Electrochemical measuring device for the continuous determination of the oxygen activity in aggressive media
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US3471390A (en) * 1965-03-24 1969-10-07 Reynolds Metals Co Alumina concentration meter

Cited By (14)

* Cited by examiner, † Cited by third party
Publication number Priority date Publication date Assignee Title
US4935107A (en) * 1986-10-17 1990-06-19 Aluminium Pechiney Process for electrochemical measurement of the concentration of oxide ions in a bath based on molten halides
EP0285578A3 (en) * 1987-03-27 1990-05-16 Centro Sviluppo Materiali S.P.A. Improvement of electrochemical devices for measuring the silicon content of hot metal
US4921584A (en) * 1987-11-03 1990-05-01 Battelle Memorial Institute Anode film formation and control
US5342489A (en) * 1989-10-17 1994-08-30 Yamari Electronite Kabushikigaisha Method of measuring oxygen activities in slag
US5294313A (en) * 1992-08-25 1994-03-15 The United States Of America As Represented By The United States Department Of Energy Sensors for monitoring waste glass quality and method of using the same
US6010611A (en) * 1995-09-01 2000-01-04 Auckland Uniservices Limited Measurement of alumina in reduction pots
US5989408A (en) * 1996-12-18 1999-11-23 Heraeus Electro-Nite International N.V. Process for measuring an electrochemical activity
US6156174A (en) * 1996-12-18 2000-12-05 Heraeus Electro-Nite International N.V. Immersion sensor for measuring an electrochemical activity
WO2000054040A1 (en) * 1999-03-05 2000-09-14 Heraeus Electro-Nite International N.V. Immersion sensor, set-up of measuring instruments and measuring method for monitoring aluminium electrolysis cell
EP1037042A1 (en) * 1999-03-05 2000-09-20 Heraeus Electro-Nite International N.V. Submergable sensor, mesuring device and methode for monitoring a aluminium electrolytic cell
US6451186B1 (en) 1999-03-05 2002-09-17 Heraeus Electro-Nite International N.V. Immersion sensor for monitoring aluminum electrolytic cells
US6620309B2 (en) 1999-03-05 2003-09-16 Heraeus Electro-Nite International N.V. Method for monitoring aluminum electrolytic cells
US20170145574A1 (en) * 2014-06-19 2017-05-25 United Company RUSAL Engineering and Technology LLC Method for controlling an alumina feed to electrolytic cells for producing aluminum
US10472725B2 (en) * 2014-06-19 2019-11-12 United Company RUSAL Engineering and Technology Centre LLC Method for controlling an alumina feed to electrolytic cells for producing aluminum

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Publication number Publication date
HU191839B (en) 1987-04-28
FR2589579A1 (en) 1987-05-07
FR2589579B1 (en) 1990-06-29
HUT37988A (en) 1986-03-28
DE3535754A1 (en) 1987-04-16

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